Power distribution systems typically include distribution feeders (ranging from approximately 4 KV to 69 KV) originating in power distribution substations and leading to the source of supply for end customers of an electrical supply utility or agency. Regulatory service provision requirements, cost and competitive pressures create requirements for lower cost, modular, standardized equipment, which can be installed, operated and maintained with minimal labor and human supervision.
Failures of the distribution feeder (faults) occur due to downed power lines, excavation of underground cable or other causes and are typically detectable by sensing excess (short circuit/overcurrent) current, and occasionally by detecting loss of voltage. In distribution systems, it is sometimes the case that a loss of voltage complaint by the customer is the means by which the utility senses the outage in order to respond by dispatching a crew to isolate the fault and reconfigure the distribution system. The typical devices for isolating these faults are circuit breakers located primarily in distribution substations and fuses located on tap lines or at customer transformers. The substation breakers are generally provided with reclosing relays that cause the breaker to close several times after the breaker has detected an overcurrent condition and tripped open. If during any of these “reclosures”, the fault becomes undetectable, service is restored and no extended outage occurs. Particularly on overhead distribution lines, temporary arcing due to wind, lightening, etc causes many faults. Thus, the majority of faults are cleared when the breaker opens and service is restored on the automatic reclose. Alternatively, after some number of reclosure attempts, if the overcurrent condition continues to be present, the recloser goes into a “lockout” state which prevents further attempts to restore service.
Although utility acceptance of more sophisticated automation solutions to fault isolation and reconfiguration has been limited but continues to increase, many methods have been developed and marketed. The most primitive methods have typically involved placing control equipment and switchgear at strategic points in the power distribution grid and coordinating their operation entirely with the use of circuit parameters sensed and operated on locally and independently at each point. More sophisticated methods have been developed for isolating/reconfiguring these circuits by communicating information sensed locally at the strategic points to a designated, higher level control entity(s). Utilizing intelligent, distributed control methodologies, several methods have been developed to isolate/reconfigure distribution circuits without the need for the higher-level control entity(s). In systems implementing these methods, information is sensed and processed locally, acted on as much as possible locally, and then shared with other cooperating devices to either direct or enhance their ability to take action. Examples of these methods include versions of the IntelliTEAM® product available from S & C Electric Company, Chicago, Ill.
Systems, such as the IntelliTEAM® products and the systems described in commonly assigned U.S. Pat. No. 6,697,240, the disclosure of which is hereby expressly incorporated herein by reference, provide methodologies and related system apparatus for using and coordinating the use of information conveyed over communications to dynamically modify the protection characteristics of distribution devices (including but not limited to substation breakers, reclosing substation breakers, and line reclosers). In this way, overall protection and reconfigurability of the distribution system or “team” is greatly enhanced. Devices within the system recognize the existence of cooperating devices outside of the team's domain of direct control, managing information from these devices such that more intelligent local decision making and inter-team coordination can be performed. This information may include logical status indications, control requests, analog values or other data.
Still, when restoration systems reconfigure distribution feeders, for the purpose of fault isolation and/or load restoration, the coordination between fault protection devices, such as interrupters, used to segment the feeder, can be destroyed. Thus, an automated method to reconfigure the protection settings to maintained coordination is desired. Further, benefits may be obtained where the fault protection devices are set so they coordinate with other devices that may or may not automatically set, for example, boundary devices such as circuit breakers that protect the distribution feeder and fuses that protect loads that are tapped off the feeder.
In some instances, where a fault current exceeds a melting current of a fuse protected distribution line, fault interruption followed by automated restoration may lead to service restoration if the fault is transient; however, service disruptions followed by inevitable service loss with the fuse melting may be the result for persistent faults or sufficiently high current faults. This may be particularly true for loads extending from the distribution line between the fuse and the fault protection device.